I don't think it's obvious. I think it WAS obvious that BMac wanted to upgrade the top-6 in the offseason and the only way to do it was by clearing some significant salary cap room. Mantha was the obvious/only choice.
Situation has changed since then. This team has shown quite clearly that they are not a minor top-6 upgrade away from playoff contention. At the same time, with Backstrom vacating the premises, there is now a whole other, much easier way to clear ample cap room.
The only thing that hasn't changed is Mantha's value. If Caps could have gotten rid of him by paying someone ~3rd round pick, maybe 3rd+4th, they would have done it ages ago. For something to change, he's got to put up points. Preferably lots of points.